This application claims the priority of Swiss Patent Application Serial No. 2001 0284/01, filed Feb. 17, 2001 the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a hydraulic oscillator as a drive of machines.
Many working machines, such as are used, for example, for punching, nibbling, embossing, hammering, forming, injection moulding or diecasting, are distinguished by rapid cyclic movements. In a punching machine, the force of the tool drive essentially determines the maximum size of the tools or the maximum thickness of the machinable material. The speed of the drive, in turn, influences the number of strokes per minute, this being an important characteristic with regard to productivity.
Owing to the stringent force and speed requirements, the tool drives of such machines are often designed hydraulically. Thus, in conjunction with high-speed valves and an electronic control, important process parameters, such as, for example, the speed profile, can be adapted to the requirements of the working process in a flexible way.
Valve technology has proved appropriate in the field of controlled hydraulic drives. In a typical solution to the above object, the working cylinder is equipped with a control valve or servovalve. The speed of the cylinder can thus be controlled accurately. A desired speed profile can be imparted to the cylinder. In addition to conventional control valves or servovalves, there is also quite a number of special valves which have been developed for this type of machines.
The solution with valves has the disadvantage that considerable throttle losses occur at the control edges of the valves. These losses heat up the hydraulic medium and, over time, lead to high energy or operating costs of the machine. For this reason, new solutions have recently been discussed. One advantageous solution is a direct drive by means of a variable-speed hydraulic pump (European Patent Application 96 913 422.0 ). The cylinder is in this case connected directly to the pump. The oil stream is controlled by the pump or by the drive motor of the pump and no longer by the valve. The valve consequently becomes superfluous as a hydraulic control element. The task of power control is assumed for the first time by electronic power actuators, in particular power transistors. These components are contained, as a rule, in the converter which supplies the variable-speed electric motor with current. The hydraulics in this case now assume only the function of a hydrostatic transmission, while control of power is performed by electrical engineering, in particular by the converter and the electric motor.
The replacement of hydraulic elements, such as variable displacement pumps or valves, by electronic power transistors has many advantages. In addition to the energy savings, this procedure makes it possible to achieve a markedly better control quality, thus, inter alia, higher reproducibility and higher thermostability, along with a markedly lower generation of noise.
On the other hand, this solution also has the disadvantage that, as a rule, the electric motor has pronounced inertia which, during cyclic movements of high frequency, has to be accelerated and decelerated correspondingly frequently. This greatly restricts the number of strokes capable of being achieved by such drives and leads to a substantial proportion of the motor power being used in order to overcome the rotational inertia of the motor. Only part of the motor power is therefore available for the actual working process.
The higher the frequency of movement is, the more motor power is used to overcome the motor""s own inertia. In an extreme case, during the entire cycle, the motor follows the predetermined speed profile with full torque, without performing any outwardly directed work. This state is reached at the maximum number of strokes capable of being achieved by the drive. Energy is then no longer delivered outwards, and the windings of the motor are constantly loaded with the maximum current and heat up the latter correspondingly.
It would therefore be desirable and advantageous to provide an improved high-speed cyclic drive for hydraulic machines, which obviates prior art shortcomings and operates without control valves, is controlled by electronic power transistors and follows a cyclic speed profile, whilst at the same time having a markedly lower power requirement. The lower drive-specific power requirement is to make it possible to implement larger numbers of strokes, along with a lower energy consumption, or to make available a greater share of power for the working process in the machine.
It would also be desirable and advantageous to design such a drive as a drive module which can be used for a multiplicity of machines and drive tasks. The invention is therefore to be interpreted in general terms as a drive of a machine joint.
According to one aspect of the present invention, two hydraulic cylinders act on the machine joint to be driven. They form four hydraulic chambers which are linked hydraulically to the machine joint. One chamber per cylinder is connected to one of the two ports of a hydraulic pump. The other chamber of the two cylinders each are connected to an energy accumulator. The pump is coupled to an electric motor and is driven by the latter.
The hydraulic pump acts in a closed hydraulic circuit on the two connected cylinder chambers. When the pump is driven by the electric motor in one direction, the machine joint moves in one direction. A reversal in direction of the electric motor results correspondingly in a reversal in direction of the machine joint.
The two energy accumulators exert the machine joint with a potential force driving back the machine joint. In general, this force increases with an increasing travel of one cylinder and decreases correspondingly in the opposite cylinder. If the energy accumulators are designed as pressure accumulators, the potential forces are pressure forces on the pistons of the cylinders. The energy accumulators may also be designed, for example, as elastic springs.
The two energy accumulators act like two springs on both sides of the load mass and turn the system into an oscillator. The speeds of the hydraulic pump and the electric motor are coupled hydraulically to the load mass. Accordingly, the load mass, together with the rotational inertia moment of the pump and motor, is to be considered as a single mass inertia between two springs. This applies at least in a first approximation if the retained oil is not considered to be compressible.
If this system is left, in the deflective state, to the free play of the forces, a natural oscillation of the spring/mass system occurs, which is defined essentially by the two energy accumulators and also all the masses and rotational inertias. This natural oscillation is damped by means of the hydraulic and mechanical power loss of all the components. In such an oscillator according to the invention, energy is exchanged between the energy accumulators and the machine joint during movement. Pressure energy in the pressure accumulator accelerates the machine joint and is consequently converted into kinetic energy of the machine, pump and electric motor. During deceleration, in turn, kinetic energy is converted into potential energy.
The forces of the motor can be controlled accurately and highly dynamically via the power electronics. By means of the device, therefore, a position can also be held or any desired speed profile can be adopted. In general, the potential forces of the energy accumulators act as additional loads on the motor which, as the case may be, assist or resist the movement of the machine joint. During an oscillating movement, the forces of the energy accumulators have, in the case of a suitable design, an assisting effect, that is to say lead to a reduction in the mean application of current to the motor, compared with the same movement cycle without potential forces of the energy accumulators.
If the movement cycle of the machine coincides with the natural oscillation of the device, the motor has to apply only minimal power losses and the power exerted outwards by the machine. Ideally, therefore 100% of the drive power is fed as useful power to the working process, leaving frictions and hydraulic losses aside. This is in complete contrast to a system according to the prior art which in the worst case (at the maximum achievable frequency) requires the entire motor power for accelerating the motor and can deliver 0% of the drive power outwards. This also applies when hydraulic losses in the lines or frictional losses in the components of the device are disregarded.